"Is the water going to be safe?" asked one of the 120 people at the meeting on ground water contamination at Howard County's Alpha Ridge Landfill in Marriottsville.

Toxins have not turned up in tests conducted by the county of selected residents' wells nearby. But last night, Ethel Doll, whose property borders the landfill, said a July 1991 test of her well showed traces of a contaminant.

Donald L. Gill, a University of Maryland Medical School professor of biochemistry and landfill activist, said the chemical found in Ms. Doll's well, tetrachlorethene, a common dry cleaning chemical, turned up in high levels in samples taken from the bedrock monitoring well.

Other chemicals found in the samples included methelene chloride, a chemical used for stripping paint; and trichloroethene, a substance used to clean grease from machinery. Each is a suspected carcinogen.

Dr. Gill noted that many people in the area use filtration systems, and that "organic compounds are taken out very easily with carbon filters."

Public Works Director James M. Irvin said subsequent tests of Ms. Doll's well turned up nothing.

Residents were given an overview of the problem, starting in 1990 with the first discoveries of volatile organic compounds (VOCs) in shallow wells north and west of the 13-year-old clay-lined landfill cell.

"This class of chemicals is commonly observed in ground water that emanates from landfills," explained Robert M. Cohen, principal hydrogeologist for GeoTrans Inc., which was awarded a county contract in November to study ground water contamination at the landfill.

In September, tests conducted by the county showed high levels of VOCs in bedrock, which residents had previously been told would be a barrier to contamination of their drinking water.

Mr. Cohen described in detail how his company would attempt to find water-bearing rock fractures and then tap them with deep wells for testing.

If needed, the wells could be used to pump out contaminated water for treatment.

Answering another resident's question, he said that climate changes or a large water user could cause ground water to change direction.

County Councilman Charles C. Feaga said that to keep residents informed, the county will send a monthly or quarterly newsletter about the landfill study to people who signed up for it last night.